Ethylene is a ubiquitous building block in the manufacture of a wide variety of chemical and plastic products. Ethylene is typically produced industrially by pyrolysis of hydrocarbons in a furnace in the presence of steam. The furnace effluent stream comprising a range of components is typically cleaned up, dried to remove water, compressed and passed to an olefins recovery section to condense the ethylene and other condensable heavy end components (ethane, propylene, propane, etc.). The condensed stream is then distilled to remove the light ends (methane and hydrogen) and fractionated to separate ethylene from the heavy ends.
Compositional range of the furnace effluent stream depends on several factors including the type of hydrocarbon feedstock used. A representative composition of the effluent of a furnace employing three different hydrocarbon feedstocks and operated to maximize ethylene formation is given in Table 1.
TABLE 1 ______________________________________ Effluent Composition (mole %) Furnace Feedstock Component Ethane Propane Naphtha ______________________________________ H.sub.2 35.9 20.5 15.8 CH.sub.4 6.5 27.8 26.5 C.sub.2 H.sub.4 34.3 32.0 33.6 C.sub.2 H.sub.6 + 23.3 19.7 24.1 ______________________________________
As can be seen, hydrogen is a substantial portion of the effluent. Hydrogen has an undesirable impact on the stream dew point temperature. Greater refrigeration power is required to condense out ethylene and other components from streams containing a high hydrogen concentration, and refrigeration makes up a significant portion of the process energy requirements. Additionally, in existing plants ethylene refrigeration availability may be limited and therefore a process bottleneck to any increase in ethylene output. U.S. Pat. No. 5,082,481 to Barchas et al. describes the use of a membrane separator to remove approximately 20 percent of the hydrogen from a cracked gas effluent containing olefins after compression but before any refrigeration of the effluent stream to separate out low-boiling components, usually before drying and removal of heavy hydrocarbons from the effluent stream well upstream from the low temperature separation system.
Membrane based unit separation systems are an advancing art. Recent developments include long lasting, high flux membrane structures. Membranes are used in gas and liquid purifying systems such as desalination, blood dialysis, recovery of precious materials from waste streams, concentrating heat sensitive biotechnical substances, and the like. Other applications proposed include industrial gas separation, processing aqueous waste streams for pollution control and processing food and beverage streams. Weber et al., Chemical Engineering Progress, November 1986, pp. 23-28 gives an overview of membrane separation systems finding application in the petrochemical and other industries including the recovery and recycle of hydrogen from purge streams in ammonia and methanol manufacture, and separation of carbon dioxide from natural gas, etc.
A least two types of membranes are commercially available, hollow fiber type membranes and spiral wound type membranes. The hollow fiber membrane is said to consist of millions of thin, hollow polymer fibers. Gases with high permeation rates diffuse through the membrane, flow out through the hollow fiber interior and are channeled into a permeate stream. Gases with a low permeation rate flow around the walls of the fibers. The driving force for the separation is the difference in partial pressure between the object gas in the feed stream and that of the permeate stream. The spiral wound membranes are similar but are made by winding the polymer on a perforate tube to form the membrane.
Davis, "Facilitated Transport Membrane Hybrid Systems for Olefin Purification," published by BP Research of Ohio in conjunction with Catalyst Consultants Inc. of Pennsylvania, November 1991, describes the use of a reverse osmosis membrane in a hybrid membrane-distillation process for separating propylene from propane. The membrane system is said to have a high flux, high selectivity and long life for operation at high transmembrane pressure.
U.S. Pat. No. 3,864,418 to Hughes et al. describes the preparation of hydrophilic, semi-permeable film membranes having a relatively large quantity of pores and containing complex-forming material dissolved in a solvent said to be useful for separating olefinically unsaturated hydrocarbons from mixtures.
As far as applicant is aware, in an ethylene plant employing hydrocarbon pyrolysis, it has been heretofore unknown to use a membrane device to reject at least a portion of the hydrogen byproduct from a reactor effluent stream after refrigeration with a low level refrigerant to partially condense out olefins from the effluent stream, but prior to final olefins recovery in the chilling train with a high level refrigerant, for the purposes of enhancing the hydrogen rejection rate (due to a higher partial pressure of hydrogen following partial olefin condensation), raising the effluent stream dew point temperature, lowering refrigeration energy usage and shifting cooling requirements from ethylene refrigeration to propylene refrigeration.